Work machines equipped with hydraulic braking systems may at times struggle to manage the interaction between the braking force provided by the braking system in response to an operator input at a braking input device, such as a brake pedal, and engine braking force that is applied to the wheels through a transmission that downshifts when the operator eases off on an input speed control, such as a gas pedal. The hydraulic braking system responds to the operator input with braking force that is proportional to the displacement of the braking input and will result in a natural feeling braking response for the operator. However, when an engine braking force is added, the work machine decelerates at a greater rate than expected, particularly for inexperienced operators, and thereby producing an unnatural feel for the operator.
Though also present in work machines having gear-type transmissions, the unnatural braking phenomenon may be particularly acute in a work machine equipped with a variable hydrostatic transmission. An example of a work machine having a hydrostatic transmission providing a braking force to the traction devices of the work machine is provided in U.S. Pat. Appl. Publ. No. US 2013/0104532 A1, published for Ries et al. on May 2, 2013, entitled “Hystat Drive System Having Coasting Functionality.” In the publication, a drive system for a machine may have an engine, a pump driven by the engine to pressurize fluid, a motor connected to the pump via an inlet passage and an outlet passage, and a traction device driven by the motor. The drive system may also have an operator input device movable from a neutral position through a range to a maximum displaced position to affect a speed of the engine, and a controller in communication with the input device and at least one of the pump and motor. The controller may be configured to gradually adjust a displacement of the at least one of the pump and motor to slow the traction device over a period of time after the operator input device is returned to the neutral position. The machine may also be equipped with an electro-hydraulic (EH) braking system having a hydraulic actuated braking device operatively associated with one of the traction devices of the work machine and providing a braking force to the traction device when commanded to do so by a controller in response to a braking signal received from a braking input. The publication teaches a method for controlling the drive system to utilize braking forces of the EH braking system and the hydrostatic transmission.
The combination of the EH braking system and the hydrostatic transmission is desirable so that the work machine can command a desired speed reduction by actively scaling the braking force versus the transmission retarding force to yield a natural feel during braking. However, EH braking systems are very expensive to implement due to redundancies that must be designed into the system to ensure that an acceptable level of braking can be achieved if an electronic component of the braking system fails during the braking cycle. In view of this, opportunities exist for providing a more economical system for combining braking forces and transmission retarding forces with a natural feel while also ensuring required minimum levels of braking in the event of an electrical fault in the braking system.